The present invention generally relates to a frame synchronizing method and apparatus for use in a television system in which the input and the output sampling rate is locked to the timing pulses associated with a master signal. The invention has broad application in television systems in general and, in particular, may be advantageously used in the chrominance time-compressed, luminance bandwidth-reduced television system disclosed in copending application Ser. No. 697,535.
Frame synchronizers are of general use in television systems whenever an incoming signal has synchronization timing which does not match the master timing of the system. This is the case, for example, when a remote broadcast is received at the broadcasting studio. Frame synchronization is necessary so that the vertical intervals of the master and remote signals coincide in time. Otherwise, switching from master to remote will cause a change in the position of the vertical interval and the image produced by television receivers receiving the signal from the studio will undergo a vertical roll. Devices such as video disk players and video tape players also require time base correction in order to provide a completely standard signal to the television system.
The chrominance time-compressed, luminance bandwidth reduced television system disclosed an application Ser. No. 697,535 also requires frame synchronization. In that system, the chrominance information is transmitted in time-compressed form during the normal horizontal retrace time. Chrominance information for a given program is transmitted once every other line, with chrominance information for a second program occupying the unused alternate line positions. Luminance information for both programs is transmitted during the active video time for each line. This is done by modulating one program on the main carrier and modulating the second program on a subcarrier approximately 4.5 MHz removed from the main carrier. Consider that the horizontal retrace in the signals for both transmitted programs must occur coincidently such that chrominance for one picture or the other can be sent during the horizontal retrace interval and the luminance information for both signals can be combined and transmitted simultaneously. Any timing misalignment would cause at least some of the luminance information from the picture in one program to be present with at least some of the chrominance in the picture for the other program. It is also necessary to synchronize the vertical intervals of both pictures so that they occur simultaneously because special processing must be allowed during the vertical interval. For example, during the vertical interval, a signal is transmitted which represents a zero or black level associated with the chrominance and luminance of each picture. Also, vertical interval reference (VIR) signals are transmitted during the vertical interval, typically on line 19 of the frame. The VIR signals are used to control the phase of an oscillator and the gain of an amplifier in order to maintain color accuracy and saturation in the picture produced by the television receiver. Further information on this system may be had by reference to U.S. Pat. No. 3,950,780 issued to Harry T. Freestone for "VIR Chroma System" and assigned to the instant assignee. Since the vertical interval is different in format, it cannot be present at the same time as luminance and chrominance information for either picture. This requires that the vertical intervals for both pictures coincide in time.